C. albicans is a successful fungal commensal, highly adapted to survive on host surfaces such as mucosal tissue where it asymptomatically colonizes epithelial surfaces. C. albicans can also cause severe opportunistic infections particularly in immunocompromised patients ranging from mucocutaneous candidiasis to bloodstream infections. Even with optimal medical care there is still substantial mortality and morbidity associated with invasive fungal disease. In order to develop new therapeutic modalities directed at fungal pathogens a detailed understanding of the innate and adaptive immune pathways involved in control of pathogens such as C. albicans are required. In this proposal we will examine the role of the nucleotide-binding domain leucine-rich repeat containing receptor (NLR) family member Nlrp3 in recognition and response to infection with C. albicans. In macrophages Nlrp3 is activated in a multiprotein complex called the inflammasome in response to a wide variety of stimuli. The activation of the Nlrp3 inflammasome ultimately results in the activation of the cysteine protease caspase-1 and its processing and secretion of proinflammatory cytokines. C. albicans activates caspase-1 in an Nlrp3-dependent manner leading to the release of interleukin (IL)-12. Nlrp3-deficient mice also demonstrate increased susceptibility to infection with C. albicans in vivo. This proposal outlines three novel aims that will examine the molecular mechanism involved in activation of Nlrp3 by C. albicans. In Aim 1 the initial priming step required for Nlrp3 inflammasome activation in response to C. albicans will be addressed by determining if signaling mediated through Syk/Card9 is required for Nlrp3 inflammasome activation. In Aim 2 we will utilize gene targeted mice to determine if reactive oxygen species and cathepsin B play a role in C. albicans-induced activation of the Nlrp3 inflammasome. We will also identify Candida specific factors expressed during yeast-hyphae transition that are directly sensed by the Nlrp3 inflammasome. In Aim 3 we will examine how the Nlrp3 inflammasome shapes subsequent adaptive immune responses following in vivo infection with C. albicans and in particular if the development of IL-17 producing Th17 cells is dependent on the presence of Nlrp3. Successful completion of the proposed studies will provide a molecular understanding of how C. albicans activates the Nlrp3 inflammasome, and will substantially augment our knowledge of how the immune system controls fungal pathogens. Furthermore, new insights into the pathogenesis of C. albicans that result from these studies may suggest novel therapeutic approaches to combating this pathogen. PUBLIC HEALTH RELEVANCE: Candida albicans is a fungal pathogen that causes a wide variety of opportunistic infections, particularly in hospitalized patient populations where it is responsible for significant morbidity and mortality. Understanding how this pathogen interacts with and evades the immune system will help us identify new therapeutic targets to treat fungal infections and is hence directly relevant to public health. Our studies will focus on how the cytosolic pattern recognition receptor Nlrp3 is activated by Candida albicans and how this shapes subsequent immune responses by the host. In addition the information gained from the proposed studies will also shed light on the pathogenesis of other fungal organisms.